[[Image:SbandAntennaMountKit.jpg|480px|thumb|Improvised antenna mount kit from the local hardware store.]]

=== Low Noise Amplifier ===

=== Low Noise Amplifier ===

Revision as of 23:00, 4 September 2009

This page documents my attempts to receive the S-band downlink of LRO and LCROSS using mostly what I already have in stock. This includes using the USRP and GNU Radio as software receiver. It is important to note that we are only talking about detecting the presence of the signal from the spacecraft and not decoding the signal. Decoding would require a much higher G/T than what can be achieved with small antennas.

System Noise Temperature

I do not have any inline devices between the antenna feed and the LNA. In fact, I will mount the LNA directly on the antenna feed via an adaptor that has a loss of ~0.1 dB.

The LNA is a HEMT-based amplifier with gain 30 dB and typical noise figure 0.5 dB corresponding to an LNA temperature of 35.4 K.

From the LNA I will have a few meters of low loss coax running to the receiver. Between the receiver and the cable coming from the LNA there is a bias-T I will use for injecting DC voltage into the coax to feed the LNA. This has an insertion loss of 1 dB.

The tuner is a wideband receiver covering 800 MHz to 2.4 GHz and has a typical noise figure between 3-5 dB. I used 4 dB which is roughly 400 K.

The total loss between LNA and receiver, including all connectors and bias-T is estimated to 3.2 dB.

The sky temperature is the noise coming from the sky. Empty regions of the sky have a temperature of 2.73 K (the cosmic background radiation), but since I will be pointing my antenna at the moon I will have to include that too.

I will use a 60 cm dish, which has a field of view (FOV) of 15.4°. The size of the Moon is ~0.52° meaning that it covers 3.4% of the antenna FOV. I will assume 250 K for the part of the sky covered by the Moon and 2.73 K for the rest:

T(sky) = 0.034 * 250 + .966 * 2.73 = 11.14K

Of course, this is just an approximation and proper calculation would use integral calculus and take the antenna pattern and temperature variations over the sky into account. For this experiment, the above approximation is good enough. Also note that I assumed that the Sun is not in the field of view of the antenna. If it was, the performance would be degraded by ~2.5 dB (measured value).

The result above is not complete - we should add about 12 K for noise coming from the atmosphere [5]:

T(sky) = 23 K

An additional contribution to the sky temperature that we have to take into account is the terrestrial noise coming into the system, for example via the side lobes of the antenna. I decided to leave that contribution at 0 K for now because I do not expect any interference at the receiving site.

A system noise temperature of 65 K is excellent for amateur equipment. Interestingly, the assembly instructions of the antenna mention that when using a good LNA one can expect a system noise temperature around 90 K that includes about 25 dB "spillover" that I didn't take into account. The 25 K difference is not much considering that the system noise temperature can quickly increase by several hundred degrees if using a lossy coax cable or bad connectors.

Detailed Component Descriptions

To be written.

Antenna

The antenna consists of a 60 cm dish equipped with a patch feed. It is the S-band antenna system by James Miller. The dish and patch have been specially manufactured for satellite operations and has better performance and e.g. standard satellite TV dishes in term of side-lobes. The gain of the antenna is 21 dBic. Comparing with the formula for a parabolic dish, we can see that the aperture efficiency is around 60%, which is very good.

Specifications

Frequency

2250-2450 MHz

Gain

21 dBic

-3dB beamwidth

16°

-10 dB beamwidth

28°

SWR

< 1.2:1

Axial ratio

1.05:1

Polarisation

RHCP

Connector

N-male [Option: N-female]

Impedance

50 ohm

Overall diameter

590 mm (23")

Weight

1.4 kg (3 lb)

Dish

The dish has 590 mm diameter, 119 mm deep, 1.2 mm thick (18swg). This gives an f/d ratio of 0.31, and is virtually identical to the dish described in Oscar News issue No. 100, Amsat Journal, Amsat-DL Journal, and many others as A 60 cm S Band Dish Antenna. The dish can be ordered from G3RUH. Package deals include the RHCP/LHCP patch feed and mounting kit.

The maximum usable frequency of the dish is TBD.

Feed

The feed can be used as an antenna on its own having a gain of 8.5 dBic. It is suitable for illuminating a dish with f/d ration between 0.3 and 0.5, therefore it works very well as a feed for the 60 cm dish. The feed polarization is LHCP (RHCP as option), so that the antenna becomes RHCP when the patch is mounted as a feed for a dish.

Specifications

Frequency

2250-2450 MHz [Option 2150-2350 MHz]

Gain

8.5 dBic

-3db beamwidth

85°

-10 dB beamwidth

125°

SWR

< 1.2:1

Axial ratio

1.05:1

Feed polarisation

LHCP [Option: RHCP]

Suitable dish f/d

0.3 to 0.5

Connector

N-male [Option: N-female]

Impedance

50 ohm

Overall diameter

120 mm

Depth

17 mm excl. connector

Weight

130 grams

Note that the gain of the patch feed can not be added directly to the gain of the dish. The gain of the dish is determined by its size. The efficiency of the feed – in particular how it illuminates the dish – has influence on the antenna efficiency, which of course has influence on the effective gain of the antenna.

Mast and mount kit

Low Noise Amplifier

Kuhne KU LNA 222 AH HEMT super low noise amplifier is mounted close to the antenna feed and is used to improve the receiver performance by increasing the figure of merit (G/T).

Expected performance gain TBD.

Specifications

Type

KU LNA 222 AH-HEMT

Frequency range

2200 ... 2400 MHz

Noise figure @ 18 °C

typ. 0.5 dB @ 2250 MHz

Gain

typ. 30 dB

Output IP3

typ. 27 dBm

Supply voltage

+9 ... 15 V DC

Current consumption

typ. 80 mA

Input connector

SMA-female, 50 ohms

Output connector

SMA-female, 50 ohms

Case

milled aluminium

Dimensions (mm)

73 x 30 x 20

Weight

90 g

The LNA will be mounted directly onto the dish feed.

Bias-T

Kuhne KU BT 271 N 10–3000 MHz bias-T is used to inject DC supply voltage needed by the LNA into the coax cable (thereby save a DC cable from shack to antenna).

Specifications

Type

KU BT 271 N

Frequency range

10 ... 3000 MHz

Insertion loss

typ. 0.1 dB @ 150 MHz
typ. 0.5 dB @ 1300 MHz
typ. 1.0 dB @ 3000 MHz

Voltage range

0 ... +15 V DC

Current

max. 1 A

DC connector

DC socket 2.1 mm

Input connector (DC output)

N-female, 50 ohms

Output connector

N-female, 50 ohms

Case

German Silver

Dimensions (mm)

37 x 37 x 30

Weight

90 g

RF Front-end (tuner)

This component is the RF daughter-board that plugs onto the USRP. It converts the high frequency RF signal to I/Q baseband that is is passed to the USRP ADCs. The options for this include the RFX2400 and DBSRX.

RFX2400

Initially, this option was considered; however, since the RFX only covers 2.3 to 2.9 GHz it is not suitable for this experiment. Even if it was possible to go down to 2.25 GHz, the noise figure of this receiver is worse than the DBSRX.

DBSRX

The DBSRX is a 800 MHz to 2.4 GHz receiver with a 3-5 dB noise figure and a software controllable channel filter that can be programmed between 1 MHz and 60 MHz[6].

It contains an MGA82563 wide band LNA followed by a MAX2118 DBS direct conversion tuner chip, followed by an AD818x (TBC) VGA. Note that according to the MAX2118 data sheet, the tuner is specified to work in the 850-2175 MHz range.

DBSRX block diagram (incl gains, AGC, dyn range)

DBSRX schematics

DBSRX PCB

MGA82563 data sheet

MAX2118 data sheet

AD818x data sheet

USRP

Software Receiver

The software receiver is implemented using GNU Radio. Since we only want to detect the signal (but not decode), the signal processing can be very sinmple and consist of some basic filtering, down-sampling and display of the baseband data coming from the USRP.

Wiring

Adaptors

Detecting the Signal

Without any detector, we can only observe the spectrum of the passband. How do we know that we are actually receiving the spacecraft and not just some background noise?

There are two possibilities:

Indirect method — Point the antenna towards the moon; we should see increased noise level in the passband. to check whether it is satellite signal or moon-noise, tune to a frequency where the satellite is not transmitting.

Direct method — We can try to demodulate (maybe even decode) one of the sub-carriers.